1. Field
The present invention relates to technology for data storage.
2. Description of the Related Art
Semiconductor-based p-i-n diodes are known in the art. These diodes are referred to as p-i-n devices because they include a region that is heavily doped with a p-type conductor (p+ region), an intrinsic region, and a region that is heavily doped with an n-type conductor (n+ region). The intrinsic region is not intentionally doped, but may have a low level of n-type and/or p-type impurities. The p-i-n diode may be formed with materials such as silicon, germanium, silicon germanium, etc. Suitable dopants can be used in the p+ region and the n+ region.
Semiconductor-based p-i-n diodes have a variety of uses. One proposed usage is a memory cell. Published U.S. Patent Application 2005/0052915 titled, “Nonvolatile Memory Cell without a Dielectric Antifuse having High- and Low-impedance States,” filed on Sep. 29, 2004 describes a p-i-n diode that has at least two resistance states such that it may be used as a memory cell. As formed, the p-i-n diode may be in a high-resistance state. Application of a programming voltage may change the resistance to a low resistance state. Published U.S. Patent Application 2005/0226067, “Nonvolatile Memory Cell Operating by Increasing Order in Polycrystalline Semiconductor Material,” which was filed Jun. 8, 2005, also describes p-i-n diodes that may be used for memory cells. Both said patent applications are hereby incorporated by reference herein for all purposes.
Semiconductor-based p-i-n diodes have also been proposed for use as a steering element in a memory array that uses elements having reversible resistance-switching behavior as the memory cells. When used as a steering element, the p-i-n diode helps to control current flow in order to control which memory cells are programmed and read. A variety of materials that show reversible resistance-switching behavior may be used as memory cells. These materials include chalcogenides, carbon polymers, perovskites, and certain metal oxides and nitrides. Specifically, there are metal oxides and nitrides which include only one metal and exhibit reliable resistance switching behavior. This group includes, for example, NiO, Nb2O5, TiO2, HfO2, Al2O3, MgOx, CrO2, VO, BN, and AlN, as described by Pagnia and Sotnick in “Bistable Switching in Electroformed Metal-Insulator-Metal Device,” Phys. Stat. Sol. (A) 108, 11-65 (1988). A layer of one of these materials may be formed in an initial state, for example a relatively low-resistance state. Upon application of sufficient voltage, the material switches to a stable high-resistance state. This resistance switching may be reversible such that subsequent application of an appropriate current or voltage can serve to return the resistance-switching material to a stable low-resistance state. This conversion can be repeated many times. For some materials, the initial state is high-resistance rather than low-resistance.
In order to function well, it is desirable for the p-i-n diode to have a high forward bias current and a low reverse bias current. The diode's rectification ratio is defined as the ratio of the forward bias current to the reverse bias current at a particular bias voltage (positive and negative). A high rectification ratio is desirable. However, techniques that provide for a higher forward bias current tend to undesirably increase the reverse bias current.
It is also desirable for the forward bias currents of all of the diodes in the memory array to be approximately the same. However, there may be variations between forward bias currents of diodes in different parts of the memory array. For some conventional memory arrays these variations are systematic. The memory array is usually formed above a substrate with some diodes pointing upwards from the substrate and others pointing downwards. By pointing upwards, it is meant that the direction of the forward bias current is away from the substrate. For some conventional memory arrays, the forward bias currents of the up-pointing diodes may consistently be higher or lower than the forward bias currents of the down-pointing diodes. These differences in forward bias currents can present problems when using the diodes to control which memory cells are programmed and read. Other problems can also arise due to the differences in currents.